On Wed, Jan 24, 2001 at 09:22:11AM +1100, Michael Still wrote:
> On Wed, 24 Jan 2001, Jeff Waugh wrote:
>
> > <quote who="Michael Still">
> >
> > > I'm not sure if you can just plug two ADSL modems into a each end of a
> > > phone connection and have it work. It is certainly an interesting idea.
> >
> > That's like running two email clients at the ends of a Cat 5 cable, and
> > expecting them to route mail.
> >
> > You need an exchange.
> > (Note: The word 'exchange' in the previous sentence was not capitalised.)
>
> Surely that's like saying I can't plug 2 56k modems into a phone
> connection and have it go. I'm not saying you wouldn't have to place a
> phone call, but can you get an ADSL-like modem that dials and then runs at
> most excellent speeds?
The thing is it wouldn't work with 56k modems either. Here's a
better explanation :)
OK for some background history here is how 33.6 works
* Modem A modulates digital signal from serial port into Analouge signal
(noise) that goes down the phone line.
* Then you're local exchange demodulates this and turns it back into a
digital signal so it can be trunked across the telco network.
* The remote exchange then remodulates this to turn the digital signal
to an Analouge signal.
* Finaly the remote modem demodulates this into a digital signal which
it sends up the serial port
Because of this constant Modulation/Demodulation this limits the
speed to about 33.6k.
Then someone had a bright idea. All the ISP's where using onramp 30's
(essatnially 30 ISDN lines or 64k digital channels) to accept incoming
calls since it only requires 4 pairs of copper instead of 30. Now since
this is all digital information anyway why not skip the last two steps
above and just give the digital signal straight to the access server.
This means you skip the last mod/demod step which means you reduce
errors etc So you can ramp up the bandwidth to 56k
Now this is why you can't do 56k from modem to modem, because
neither modem has a digital connection o the special hardware required
so you can't skip the modulation/demodulation step.
OK enough intro. What's the deal with ADSL. It's the same sort
of things with a few more limitations.
* ADSL modems talk to a DSLAM which has expensive hardware that
understands the magic required to push upto 8Mbs down two strands of
copper.
* There needs to be a singal piece of copper between you and the DSLAM
ie you can't have anyother equipment inbetween eg exchanges, rims etc
* There is a 3-5km limit on distance.
Now if you are looking for an end to end solution that bypasses the
telco then you need either HDSL or CityDSL, but these both require a
singal piece of copper between both locations. There's a dstance limit
of 3km and the modems on each end range from $1k-$3k
--
John Ferlito
Senior Engineer - Bulletproof Networks
ph: +61 (0) 410 519 382
http://www.bulletproof.net.au/
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